Simply put your cursor in the large box top centre of their page and click Edit - Paste, to put the page address you copied into the box.

Then click on the down arrow at the side of the box underneath to bring up a list of languages available. Select the one you require by clicking on it, then press the grey "Translate" button next to the box of language options.

Within a minute or two this will generate the page URL
with the text translated into the language you selected.

[If you wish to translate plain text, you can also edit-copy the
block of text and then paste this into the box on the Systran page.]